The invention relates to screening devices, more precisely to equipment used for feeding screening devices, as well as to a control system of the same.
It has been known heretofore to separate fractions of different sizes from a material by screening. For this purpose, a number of different kinds of screens have been developed, and vibrating screens and trommel screens can be mentioned as examples. To facilitate the feeding of the screen and the discharge of the screened material, the screens are seldom equipped with a power transmission of their own and with a control system of their own so that the screen, the power transmission and the control alone would constitute the screening machine, but typically various feeding equipment and discharging equipment are connected to the screening machine. Such devices can be for example vibrating feeders, conveyors, pendulum feeders, etc.
In practice, the screening machines are often composed at least of power transmission, control, a screen, a feeding conveyor and a discharge conveyor. Such a simple device is capable of performing a simple screening process, starting from the feeding of the material to the screen and ending in the discharge of the screened material fractions from the screen.
Typical feed materials include various earth materials, such as gravel, quarried rock, top soil (humus) and peat, as well as various products, by-products and wastes of industrial processes.
It is also known to equip the screening machine of the above kind with various auxiliary devices that further facilitate the screening. One such a device is a shredder that comminutes the pieces In the feed material that may obstruct the holes in the mesh if they reach the screen in full size. Such pieces may include, for example, root-lumps, sticks, branches or timber
The screening machine often comprises two different discharge conveyors, wherein the accept and reject of the screen can be discharged far away from each other without their mixing with each other after the screening. If the screen is equipped with several screen decks, the screen is usually equipped with an even larger number of conveyors in such a manner that the reject of the topmost screen deck and the accept of each screen deck in the screen can be transferred away from the screening machine. Preferably the discharge conveyors are long, thus allowing the stacks of products to be conveyed as far away from the screening machine as possible. At the same time their discharge ends can be placed on a high level, wherein product heaps of large volume are attained.
Furthermore, it is known to equip the screening machine with wheels or tracks to facilitate its moving.
The power transmissions of screening machines are typically based on electric power transmission or hydraulic power transmission. The power source is typically a diesel engine, a separate electric generator or public electric power supply system.
In its simplest form the control of the screening machine is implemented in such a manner that the user starts and stops each processing unit of the screening machine separately by acting on the valves of a hydraulic circuit or the switches of an electric drive. As a rule, screening machines also contain one or more emergency stop devices typical for working machines.
More advanced devices utilize different microprocessor-based control systems wherein it is possible to facilitate the use of the machine. It is for example known to equip the screening machine with a PLC control (programmable logic controller), wherein the entire process of the screening machine can be started and stopped in accordance with programmed starting and stopping sequences with the push of one button.
It is also known to equip the processing units of the screening machine with different kinds of sensors to Indicate the status of operation of the machine to the user. For example by monitoring the operating speed of the screen itself or its input power, it is possible to determine whether the loading of the screen Is suitable in relation to its capacity.
Similarly, it is known to use sensor systems to indicate different faults in the machine to the user. By incorporating such condition monitoring sensors in the microprocessor-based control, it is possible to bring the screening machine to run down the screening process in a controlled manner in accordance with a programmed stopping sequence, for example in a situation where there is a risk of a damage, so that the machine is emptied of the material to be screened before it stops.
Other factors having effect on screening capacity include such as type of feed material, angle of the screen, area of the screen and the mesh type. These given, the main thing having effect on screening capacity is the feed capacity.
However, all known screening solutions share the same problem: it is difficult to optimize the feeding speed of the process. It demands a great deal of skill from the user of the machine to be able to adjust the feeding speed of the machine in case of varying feed material in such a manner that the maximum screening capacity could be obtained from the screening machine, and on the other hand in such a manner that the products produced by screening could be as clean as possible. Both of these objectives are significantly affected by the feeding capacity of the screen in such a manner that a feeding capacity that is normally too small produces a clean screening product of good quality, but a small production capacity. Too large a feeding capacity, in turn, normally results in a good production capacity, but at the cost of the purity of the screening.
The selection of the feeding capacity of the screening machine is a task of optimization in which the layer of feed material fed on the topmost deck of the screen must be sufficiently thick so that the screen would produce the maximum amount of screened end products. On the other hand, the user must be able to adjust the material on the screen into a sufficiently thin layer, so that the screen would not be overloaded and the purity of the screening would be maintained.
In this context the screening purity refers to that how well the different fractions are separated from each other. It is obvious for anyone skilled in the art that too thick a material layer on the topmost surface of the screen means that even some of the fractions smaller than the mesh size of the topmost screen deck travel over the entire mesh without ever passing through the mesh.
Thus, too thick a material layer also causes overloading of the screen. This causes reduction in the running speed of the screen, or in the case of certain types of vibrating screens, shortening of the vibrating movement and thus a reduction in the screening capacity. This may also cause various damages, for example damages in the power transmission means, bearings or drives, or even fatigue damages in the frame structures. Typical damages in the vibrating screen include for example damages in the springs or damages in the vibrator.
In practice, the overloading of the screen becomes evident in the hydraulic drive as an increase in the hydraulic pressure and in the electric drive as an increase in the current used by the drive motor. Irrespective of the driving method, overloading manifests itself in the worst case as a decrease in the running speed of the screen.